1. Field of the Invention
The present invention relates to a control system for a hybrid vehicle that includes an engine and either a motor or a motor generator as drive power sources, and in particular to a travel mode control system in a hybrid vehicle that is configured to be capable of disconnecting an engine from a drive power transmission system.
2. Description of Related Art
As a type of a hybrid drive device, a so-called two-motor hybrid type has been known that controls an engine speed by a first motor or a motor generator (hereinafter, may collectively be referred to as a motor) and drives a second motor by electric power that is generated by the first motor. In this type of the device, the engine speed may be controlled by the first motor while engine output is maintained to be constant. Thus, in order to allow such control, the engine and the first motor are connected via a differential mechanism. An example of this is disclosed in Japanese Patent Application Publication No. 08-295140 (JP 08-295140 A).
A configuration of the example will be briefly described. A first rotary element in a planetary gear train, which is the differential mechanism, is connected to a generator, a second rotary element serves as an output element, and a third rotary element is connected to braking means. Then, an engine is connected to the third rotary element via a clutch. A motor is connected to the second rotary element. Thus, in the configuration disclosed in JP 08-295140 A, when a first motor functions as the generator in a state that the engine is driven to run a vehicle, an engine speed is set to a speed that corresponds to a rotational speed of the first motor. In this case, the electric power that is generated by the first motor is supplied to a second motor. Torque output by the second motor is combined with torque transmitted from the engine and is transmitted to drive wheels. In addition, since the differential mechanism functions as a speed reduction gear or a speed increasing gear when the third rotary element is fixed by the engine or the braking means, the torque of the first motor, which is connected to the first rotary element, can be transmitted to the output element. In other words, the vehicle can travel by power that is output from the first and second motors. Then, as the braking means, if a one-way clutch is adopted that is engaged when the torque is applied to the third rotary element in a direction to reversely rotate the third rotary element so as to fix the third rotary element, the third rotary element can be fixed not by the engine but by the one-way clutch while the vehicle travels by the motor. In addition, the engine can be disconnected from the third rotary element in this case. Thus, it is possible to keep driving or stop the engine.